It has long been known to use electromagnetic waves in connection with absorption spectroscopy, and then particularly light beams or bundles of light rays (light bundles) whose frequencies lie within the infrared range, and to use different types of detector elements.
In the case of gas sensors that are based on absorption technology, the light beams shall be permitted to pass through a gas sample contained in a cell cavity, wherewith a given volume of a selected gas will result in frequency-dependent absorption of the light beams.
A gas detector is thus adapted to detect the frequency spectrum of the light beams for a relevant gas or gas concentration within the cavity, and the gas or gas concentration can be determined by evaluating the intensity of the detected light beam in relation to a chosen intensity for the incident light beams and the absorption coefficient of the light concerned or the electromagnetic wavelength in the gas concerned.
With regard to the features of the present invention, it can be mentioned that it is known to compress a gas cell with respect to its physical dimension, by allowing the light beams to be reflected a number of times within the cavity, thereby to obtain a relatively long optical measurement path extremity (wavy length) or a relatively long absorption path extremity relative to the internal dimensions of the gas cell or the cavity.
The absorption cell or gas cell disclosed in Patent Publication U.S. Pat. No. 5,009,493 is an example of a gas cell that is designed to provide an adapted long absorption path extremity or an optical measuring path extremity within a defined or delimited cavity, where incoming light beams (or light bundles) shall be reflected a number of times within the cavity before being exited therefrom and thereafter fall on a light detector.
In gas cells of this kind, it is usual to allow incoming light beams to pass into the cavity via a first opening, and to exit the beams from the cavity via a second opening, therewith enabling a detector that is used in the present context to consist of a separate unit that is preferably mounted on the gas cell in connection with the second opening.
The cavity of the gas cell is formed normally by at least a first and a second part, whose inner surfaces may be treated to provide surfaces that strongly reflect the incoming light beams or light bundles.
This surface treatment normally comprises coating the inner surface with one or more layers of metal, wherewith the reflecting surfaces are formed by the last metal layer applied.
The metal and procedure chosen for this coating process will depend on the desired optical qualities of the surfaces and also on the optical wavelength or wavelengths that shall be reflected by said surfaces. The material in the gas cell body shall also be taken into consideration.
With regard to the significant characteristic features and properties of the present invention, mention shall also be made to an example of an earlier known gas sensor in which incoming light beams or light bundles are reflected a number of times within the cavity in accordance with a predetermined pattern.
In this respect, reference is made to a gas sensor that is illustrated and described in International Patent Application PCT/SE96/01448, Publication No. WO97/10460 and the International Patent Application PCT/SE97/01366, Publication No. WO98/09152, from which the present invention can be considered to constitute a development.